Heat stroke is a condition in which your body becomes dehydrated and overheated, with the body temperature rising above 40.5C.
The body is usually able to dispense heat by radiation of heat through the skin or by evaporation of sweat, but when it's unable to cool itself heat stroke can result. It often happens over a fairly short period of time and can be life-threatening if proper medical attention isn't received.
The condition can be brought on through being in enclosed, hot places such as rock concerts or sporting events. Strenuous physical activity in a hot environment is also associated with heat stroke as is working in confined areas or any other conditions that raise the body temperature.